0.目录
1.
- 1.1
- 1.2
- 1.3
2.
- 2.1
- 2.2
- 2.3
- 2.4
- 2.5
3.
1.抽象类和接口
1.1 抽象类
面向对象中的抽象类:
- 可用于表示现实世界中的抽象概念
- 是一种只能定义类型,而不能产生对象的类
- 只能被继承并重写相关函数
- 直接特征是相关函数没有完整的实现
Shape是现实世界中各种图形的抽象概念,因此:
- 程序中必须能够反映抽象的图形
- 程序中通过抽象类表示图形的概念
- 抽象类不能创建对象,只能用于继承
1.2 纯虚函数
抽象类与纯虚函数:
- C++语言中没有抽象类的概念
- C++中通过纯虚函数实现抽象类
- 纯虚函数是指只定义原型的成员函数
- 一个C++类中存在纯虚函数就成为了抽象类
纯虚函数的语法规则:
示例——抽象类:
#includeusing namespace std;class Shape{public: virtual double area() = 0;};class Rect : public Shape{ int ma; int mb;public: Rect(int a, int b) { ma = a; mb = b; } double area() { return ma * mb; }};class Circle : public Shape{ int mr;public: Circle(int r) { mr = r; } double area() { return 3.14 * mr * mr; }};void area(Shape* p){ double r = p->area(); cout << "r = " << r << endl;}int main(){ Rect rect(1, 2); Circle circle(10); area(&rect); area(&circle); return 0;}
运行结果为:
[root@bogon Desktop]# g++ test.cpp[root@bogon Desktop]# ./a.out r = 2r = 314
- 抽象类只能用作父类被继承
- 子类必须实现纯虚函数的具体功能
- 纯虚函数被实现后成为虚函数
- 如果子类没有实现纯虚函数,则子类成为抽象类
1.3 接口
满足下面条件的C++类则称为接口:
- 类中没有定义任何的成员变量
- 所有的成员函数都是公有的
- 所有的成员函数都是纯虚函数
- 接口是一种特殊的抽象类
示例——接口:
#includeusing namespace std;class Channel{public: virtual bool open() = 0; virtual void close() = 0; virtual bool send(char* buf, int len) = 0; virtual int receive(char* buf, int len) = 0;};int main(){ return 0;}
C++中没有真正的接口,但是C++的后续语言Java、C#直接支持接口的概念!
2.被遗弃的多重继承
2.1 C++中的多重继承
C++支持编写多重继承的代码:
- 一个子类可以拥有多个父类
- 子类拥有所有父类的成员变量
- 子类继承所有父类的成员函数
- 子类对象可以当作任意父类对象使用
多重继承的语法规则:
2.2 多重继承的问题一
示例——多重继承的问题一:
#includeusing namespace std;class BaseA{ int ma;public: BaseA(int a) { ma = a; } int getA() { return ma; }};class BaseB{ int mb;public: BaseB(int b) { mb = b; } int getB() { return mb; }};class Derived : public BaseA, public BaseB{ int mc;public: Derived(int a, int b, int c) : BaseA(a), BaseB(b) { mc = c; } int getC() { return mc; } void print() { cout << "ma = " << getA() << ", " << "mb = " << getB() << ", " << "mc = " << mc << endl; }};int main(){ cout << "sizeof(Derived) = " << sizeof(Derived) << endl; // 12 Derived d(1, 2, 3); d.print(); cout << "d.getA() = " << d.getA() << endl; cout << "d.getB() = " << d.getB() << endl; cout << "d.getC() = " << d.getC() << endl; cout << endl; BaseA* pa = &d; BaseB* pb = &d; cout << "pa->getA() = " << pa->getA() << endl; cout << "pb->getB() = " << pb->getB() << endl; cout << endl; void* paa = pa; void* pbb = pb; if( paa == pbb ) { cout << "Pointer to the same object!" << endl; } else { cout << "Error" << endl; } cout << "pa = " << pa << endl; cout << "pb = " << pb << endl; cout << "paa = " << paa << endl; cout << "pbb = " << pbb << endl; return 0;}
运行结果为:
[root@bogon Desktop]# g++ test.cpp[root@bogon Desktop]# ./a.out sizeof(Derived) = 12ma = 1, mb = 2, mc = 3d.getA() = 1d.getB() = 2d.getC() = 3pa->getA() = 1pb->getB() = 2Errorpa = 0x7ffc9f641dc0pb = 0x7ffc9f641dc4paa = 0x7ffc9f641dc0pbb = 0x7ffc9f641dc4
通过多重继承得到的对象可能拥有“不同的地址”!!
解决方案:无 (其实pa和pb还是指向了同一个对象,但是指向的是同一个对象的不同位置,打个比方就是pa指向了这个对象的脑袋,pb指向了这个对象的胸口。。。)2.3 多重继承的问题二
多重继承可能产生冗余的成员:
示例——多重继承的问题二:
#includeusing namespace std;class People{ string m_name; int m_age;public: People(string name, int age) { m_name = name; m_age = age; } void print() { cout << "Name = " << m_name << ", " << "Age = " << m_age << endl; }};class Teacher : public People{public: Teacher(string name, int age) : People(name, age) { }};class Student : public People{public: Student(string name, int age) : People(name, age) { }};class Doctor : public Teacher, public Student{public: Doctor(string name, int age) : Teacher(name + "1", age + 10), Student(name + "2", age + 1) { }};int main(){ Doctor d("Bob", 33); //d.print(); d.Teacher::print(); d.Student::print(); return 0;}
运行结果为:
[root@bogon Desktop]# g++ test.cpp[root@bogon Desktop]# ./a.out Name = Bob1, Age = 43Name = Bob2, Age = 34
当多重继承关系出现闭合时将产生数据冗余的问题!!!!
解决方案:虚继承- 虚继承能够解决数据冗余问题
- 中间层父类不再关心顶层父类的初始化
- 最终子类必须直接调用顶层父类的构造函数
示例——使用虚继承解决数据冗余:
#includeusing namespace std;class People{ string m_name; int m_age;public: People(string name, int age) { m_name = name; m_age = age; } void print() { cout << "Name = " << m_name << ", " << "Age = " << m_age << endl; }};class Teacher : virtual public People{public: Teacher(string name, int age) : People(name, age) { }};class Student : virtual public People{public: Student(string name, int age) : People(name, age) { }};class Doctor : public Teacher, public Student{public: Doctor(string name, int age) : Teacher(name+"1", age), Student(name+"2", age), People(name+"3", age) { }};int main(){ Doctor d("Delphi", 33); d.print(); d.Teacher::print(); d.Student::print(); return 0;}
运行结果为:
[root@bogon Desktop]# g++ test.cpp[root@bogon Desktop]# ./a.out Name = Delphi3, Age = 33Name = Delphi3, Age = 33Name = Delphi3, Age = 33
问题:
当架构设计中需要继承时,无法确定使用直接继承还是虚继承!!2.4 多重继承的问题三
多重继承可能产生多个虚函数表
示例——多重继承的问题三:
#includeusing namespace std;class BaseA{public: virtual void funcA() { cout << "BaseA::funcA()" << endl; }};class BaseB{public: virtual void funcB() { cout << "BaseB::funcB()" << endl; }};class Derived : public BaseA, public BaseB{};int main(){ Derived d; BaseA* pa = &d; BaseB* pb = &d; BaseB* pbb = (BaseB*)pa; cout << "sizeof(d) = " << sizeof(d) << endl; cout << "Using pa to call funcA()..." << endl; pa->funcA(); cout << "Using pb to call funcB()..." << endl; pb->funcB(); cout << "Using pbb to call funcB()..." << endl; pbb->funcB(); cout << endl; cout << "pa = " << pa << endl; cout << "pb = " << pb << endl; cout << "pbb = " << pbb << endl; return 0;}
运行结果为:
[root@bogon Desktop]# g++ test.cpp[root@bogon Desktop]# ./a.out sizeof(d) = 16Using pa to call funcA()...BaseA::funcA()Using pb to call funcB()...BaseB::funcB()Using pbb to call funcB()...BaseA::funcA()pa = 0x7fffd5157c20pb = 0x7fffd5157c28pbb = 0x7fffd5157c20
需要进行强制类型转换时,C++中推荐使用新式类型转换关键字!!
解决方案:dynamic_cast示例——使用新式类型转换dynamic_cast关键字:
#includeusing namespace std;class BaseA{public: virtual void funcA() { cout << "BaseA::funcA()" << endl; }};class BaseB{public: virtual void funcB() { cout << "BaseB::funcB()" << endl; }};class Derived : public BaseA, public BaseB{};int main(){ Derived d; BaseA* pa = &d; BaseB* pb = &d; BaseB* pbb = (BaseB*)pa; // oops!! BaseB* pbc = dynamic_cast (pa); cout << "sizeof(d) = " << sizeof(d) << endl; cout << "Using pa to call funcA()..." << endl; pa->funcA(); cout << "Using pb to call funcB()..." << endl; pb->funcB(); cout << "Using pbb to call funcB()..." << endl; pbb->funcB(); cout << "Using pbc to call funcB()..." << endl; pbc->funcB(); cout << endl; cout << "pa = " << pa << endl; cout << "pb = " << pb << endl; cout << "pbb = " << pbb << endl; cout << "pbc = " << pbc << endl; return 0;}
运行结果为:
[root@bogon Desktop]# g++ test.cpp[root@bogon Desktop]# ./a.out sizeof(d) = 16Using pa to call funcA()...BaseA::funcA()Using pb to call funcB()...BaseB::funcB()Using pbb to call funcB()...BaseA::funcA()Using pbc to call funcB()...BaseB::funcB()pa = 0x7ffcc2c27ff0pb = 0x7ffcc2c27ff8pbb = 0x7ffcc2c27ff0pbc = 0x7ffcc2c27ff8
2.5 正确的使用多重继承
工程开发中的“多重继承”方式:
- 单继承某个类 + 实现(多个)接口
示例——正确的多继承方式:
#includeusing namespace std;class Base{protected: int mi;public: Base(int i) { mi = i; } int getI() { return mi; } bool equal(Base* obj) { return (this == obj); }};class Interface1{public: virtual void add(int i) = 0; virtual void minus(int i) = 0;};class Interface2{public: virtual void multiply(int i) = 0; virtual void divide(int i) = 0;};class Derived : public Base, public Interface1, public Interface2{public: Derived(int i) : Base(i) { } void add(int i) { mi += i; } void minus(int i) { mi -= i; } void multiply(int i) { mi *= i; } void divide(int i) { if( i != 0 ) { mi /= i; } }};int main(){ Derived d(100); Derived* p = &d; Interface1* pInt1 = &d; Interface2* pInt2 = &d; cout << "p->getI() = " << p->getI() << endl; // 100 pInt1->add(10); pInt2->divide(11); pInt1->minus(5); pInt2->multiply(8); cout << "p->getI() = " << p->getI() << endl; // 40 cout << endl; cout << "pInt1 == p : " << p->equal(dynamic_cast (pInt1)) << endl; cout << "pInt2 == p : " << p->equal(dynamic_cast (pInt2)) << endl; return 0;}
运行结果为:
[root@bogon Desktop]# g++ test.cpp[root@bogon Desktop]# ./a.out p->getI() = 100p->getI() = 40pInt1 == p : 1pInt2 == p : 1
一些有用的工程建议:
- 先继承自一个父类,然后实现多个接口
- 父类中提供equal()成员函数
- equal()成员函数用于判断指针是否指向当前对象
- 与多重继承相关的强制类型转换用dynamic_cast完成
3.小结
- 抽象类用于描述现实世界中的抽象概念
- 抽象类只能被继承不能创建对象
- C++中没有抽象类的概念
- C++中通过纯虚函数实现抽象类
- 类中只存在纯虚函数的时成为接口
- 接口是一种特殊的抽象类
- C++支持多重继承的编程方式
- 多重继承容易带来问题
- 可能出现“同一个对象的地址不同”的情况
- 虚继承可以解决数据冗余的问题
- 虚继承的使得架构设计可能出现问题
- 多继承中可能出现多个虚函数表指针
- 与多重继承相关的强制类型转换用dynamic_cast完成
- 工程开发中采用“单继承多接口”的方式使用多继承
- 父类提供成员函数用于判断指针是否指向当前对象